In an image forming apparatus, such as a copy machine, a printer, and a compound machine, based on image data which is read by a document reading device, a toner image is formed on an image carrier provided in the image forming apparatus, and after the toner image is transferred onto a recording sheet supplied from a tray, the toner image is fixed, whereby permanent image formation (or printing) is performed.
Since various types of sheets are used for printing, a coated sheet is characterized in that its air permeability is relatively low due to its coated surface, further, under a high humidity environment, moisture tends to be trapped between adjacent stacked coated sheets. Accordingly, adhesion force tends to be high between the stacked sheets, and a top coated sheet is not easily separated from a second stacked coated sheet on the tray, whereby the coated sheets are not easily supplied to the apparatus, resulting in erratic sheet feeding. Still further, when the recording sheet is conveyed under said high humidity, and printing is conducted on the sheet, paper sheet debris generated from the recording sheets accumulates in the conveyance path, interfering with proper operation of optical sensors mounted in the conveyance path to detect the recording sheets. To overcome these problems, an image forming apparatus is proposed in which a dehumidifying heater is mounted within each tray to decrease the humidity of the coated sheets. Further, in an image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-276,883, the humidity in the apparatus is detected by a humidity sensor, and if the humidity is relatively high in the apparatus, a tray featuring a long conveyance path is automatically selected, and sheet debris is controlled to not be generated in the high humidity environment, so that the optical sensors, mounted in the conveyance path which is commonly used with other trays, are prevented from being covered with said debris.
However, such dehumidification is not sufficient in some cases. For example, just after power is switched on, or during environmental variation, if coated sheets are being conveyed in the path, they tend to cause a jam, or sheet debris may accumulate on the conveyance path, which are major problems. Further, if printing is conducted on the recording sheet under high humidity, heat is absorbed by moisture in the recording sheet, and fixing temperature in the recording sheet does not reach a desired temperature, which may cause an insufficiently fixed image due to low fixing temperature.
Still further, in the image forming apparatus proposed in the above patent document, sheet debris tends to accumulate on the optical sensor mounted near the tray which supplied the sheets, tending to cause the coated sheets to jam, due to the high humidity environment.